1. Field of the Invention
This invention relates to a method and apparatus for operating a thermal drying system, and more particularly, to a system for maintaining the temperature of gas that has been passed over wet material in a drying chamber at a constant temperature, and to a system for controlling the amount of wet material contained in a storage means that supplies wet material to a drying chamber.
2. Description of the Prior Art
Thermal dryers are presently utilized in many industrial operations to remove moisture from wet material and the like that must be dried before the material can be made available to consumers or other industrial operations. Thermal dryers employ different types of operating control systems, depending upon what is going to be done with the material after the material has passed through the thermal dryer.
U.S. Pat. No. 3,396,476 discloses a method for controlling the drying of alfalfa preparatory to the alfalfa being pelletized. The dehydrator comprises a rotary dryer that receives alfalfa in one end and exposes the alfalfa to hot air from a furnace. The hot air and dried alfalfa are separately discharged from the other end. The moisture content of the alfalfa introduced into the drying chamber varies substantially, yet it is necessary that the moisture content of the alfalfa after dehydration remain essentially constant. The temperature of the air within the discharge end of the dehydrator and the temperature of the discharged alfalfa, which is indicative of the moisture content, are sensed and actuate computing controllers to increase or decrease the rate at which wet alfalfa is supplied to the dehydrator. For any average rate of feed of wet alfalfa to the dehydrator, the furnace temperature and amount of hot air supplied by the furance remain constant.
U.S. Pat. No. 3,395,459 discloses an apparatus for drying wood veneer in which hot air is supplied to a drying chamber. A conveyor system is positioned inside the drying chamber. Wood veneer sheets are moved through the drying chamber on the conveyor system, and hot air supplied by a furnace is passed over the veneer sheets to remove the moisture therefrom. A temperature sensor monitors the temperature of the hot air in the drying chamber. Variations in the hot air temperature are utilized to adjust the speed of the conveyor system, resulting in an adjustment in the amount of time the veneer sheets are subjected to the hot air within the drying chamber. The conveyor system speed is regulated so that the veneer sheets remain in the drying chamber for a sufficient period of time to dry ninety percent of the sheets.
U.S. Pat. No. 3,732,435 discloses an apparatus for measuring the moisture content of material being conveyed through a machine for conditioning the material to achieve a selected moisture content, and for controlling the speed at which the material is conveyed through the machine in order to subject the material to longer or shorter periods of exposure to the conditioner for the purpose of achieving the selected moisture content for the material.
U.S. Pat. Nos. 2,136,870; 2,323,289; 2,666,269; 3,672,070; 3,732,435; 3,783,527; 4,379,692 and 4,487,577 are directed to various types of dryers, dryer feeds rotary kiln-type dryers, fluidized bed-type dryers and the like.
It has been suggested by the prior art devices to utilize temperature variations inside a drying chamber to increase or decrease either the rate at which wet material is supplied to a drying chamber or the rate at which wet material is passed through a drying chamber, to control the moisture content of the material discharged from the drying chamber. There is a need for a method and apparatus to control the temperature variations inside a drying chamber, and more particularly, for a method and apparatus to maintain the temperature inside a drying chamber at a constant preselected temperature. The temperature inside the drying chamber is maintained at the constant preselected temperature by automatically controlling the amount of wet material introduced into the drying chamber. There is also need for a method and apparatus for controlling the amount of wet material contained in a storage means that supplies wet material to a drying chamber. The amount of wet material contained in the storage means is regulated by controlling the drying capacity of the drying chamber.